Fuel reformers can be used to convert alcohol fuels into gaseous fuels (reformates) to fuel an engine. For example, an ethanol reformer can reform ethanol into a reformate gas comprising hydrogen (H2), carbon monoxide (CO), and methane (CH4) for combustion in an engine. The hydrogen content of the fuel improves combustion stability, enabling the engine to be operated with higher dilution levels (such as with higher EGR levels), thereby improving fuel economy. One example of an ethanol reformation system is shown by Kerns et al in U.S. Pat. No. 8,539,914. Therein, a flexible fuel engine is configured with a fuel reformer which reforms a variable ethanol fuel into a gaseous fuel reformate.
The inventors herein have recognized that the fuel economy benefits of the reformation system may be limited due to, as one example, the temperature variable efficiency of the reformer. In particular, since the reformer relies on exhaust energy for driving the endothermic reformation reaction, fuel reforming may not be possible at lower engine loads when the exhaust temperature is lower. Likewise, due to the efficiency of the reformer varying with exhaust temperature, a desired engine operating point that is most optimal for the reformer may vary significantly from the actual engine operating point at the time of reformation. All of these issues result in the optimal fuel economy benefit of the fuel reformer not being realized.
In view of these issues, the inventors herein have recognized that by integrating a fuel reformation system into a hybrid vehicle system, various synergies can be achieved. In one example, potential synergies are attained by a method for a hybrid vehicle comprising: responsive to lower than threshold engine load while an available reformate is lower than a threshold level, raising engine load above the threshold load and charging a system battery using excess engine torque; and reforming a liquid fuel using exhaust energy, the engine operated with the raised load until the available reformate is higher than the threshold level. In addition, responsive to an engine shutdown request, the engine may be shutdown while fuel reformation is continued until a reformer temperature is below a threshold. In this way, fuel economy of a vehicle with an on-board reformer can be enhanced.
As an example, a hybrid vehicle may be configured with a battery powered electric motor (or motor/generator) for propelling vehicle wheels via motor torque, as well as an engine for propelling vehicle wheels via engine torque, the engine including a fuel reformer. The engine may be propelled with a first, liquid fuel such as an ethanol-gasoline blended fuel. In addition, when there is insufficient gaseous fuel available, the reformer may be operated using engine exhaust energy to reform the liquid fuel into a gaseous fuel. For example, the ethanol fuel may be reformed into methane, hydrogen, and carbon dioxide, the hydrogen content of the reformate improving engine combustion stability. During conditions when the vehicle is propelled via the engine, and the engine load is high enough to maintain elevated exhaust temperatures (and therefore elevated reformer temperatures), the reformer may be operated and the presence of hydrogen in the reformate may enable the engine to be operated with higher engine dilutions (such as with higher EGR levels). In comparison, during lower load conditions, based on the state of charge of the battery, vehicle operation may be adjusted to prolong reformate usage. For example, if the battery state of charge is lower (and the battery is capable of accepting charge), the engine output may be raised above a level required for propulsion while the excess torque is used to charge the battery. Herein, the elevated engine output raises the exhaust temperature so that the reformer can be efficiently operated while the excess torque is stored in the battery to reduce any associated drivability issues. Else, if the battery cannot accept further charge, the load at which the engine is shutdown may be raised so that the vehicle can be propelled in an electric mode instead of operating the engine in a low load mode where fuel reformation is not possible. Anytime the reformer is operated, based on the reformer conditions, the engine may be held in a narrower engine speed-load that provides optimal reformer efficiency while torque transients are maintained via the system motor/battery. This allows for increased fuel reformation at higher reforming efficiencies. In addition, following an engine shutdown request, even after an engine shut down, reformer operation may be continued while the exhaust temperature is hot enough to support the reformation. Further, a temperature at which the reformer is deactivated may be adjusted based on the amount of available reformate.
In this way, by integrating an engine exhaust driven fuel reformer with a motor on a hybrid vehicle, various synergies can be achieved, improving vehicle fuel economy. The technical effect of raising an engine load even when torque demand is low, and increasing charging of a system energy storage device is that exhaust temperatures can be maintained elevated and reformer operation can be extended. By adjusting the load threshold at which an engine is shutdown, engine operation at higher loads can be maximized while engine operation at lower loads is minimized, improving reforming benefits. By extending reformer operation after an engine shutdown responsive to exhaust temperature, reformation conditions may be extended. By enabling reformate generation over a larger range of vehicle operating conditions, engine combustion stability may be improved and EGR usage can be increased, which provides fuel economy and emissions benefits. The technical effect of using battery/motor power to hold the engine in a narrow operating range selected based on reformer conditions is that the reformer can be operated at an optimal efficiency despite changes in driver or wheel torque demand. Overall, vehicle performance can be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.